Conventionally, blood that is collected with a syringe is measured by a high-precision blood sugar analyzer in a hospital or a clinic, and blood that is collected by puncturing a finger tip or the like is measured by a portable and simple blood sugar meter at home or outside home, thereby obtaining a blood sugar level. These measuring methods are called invasive systems. Either method causes pain because a human body is punctured, leading to mental and physical drawbacks in performing measurement and management of the blood sugar level. Further, it is necessary to consider risks of infection diseases as well as disposal of wastes on which blood is attached.
So, blood sugar level measurement methods using various kinds of noninvasive techniques have been proposed. The noninvasive method measures a blood sugar level in a contact or noncontact manner, without hurting a human body, with no pain. As for measurement means, there are proposed electromagnetic wave, acoustic wave, and electrical characteristics, as well as optical rotation or glucose, and absorbance or reflectivity of infrared light or visible light. As for measurement sites, there are proposed skin surfaces of such as arm, finger, and ear, eyeball, nail, sputum, and the like.
In these measurement methods, a blood sugar level in blood vessels is not directly measured, but a biological feature amount relating to a change in the blood sugar level is measured, and it is general to prepare a correlation table between biological feature amounts and blood sugar levels. Since the change in the biological feature amount with respect to the change in the blood sugar level varies among individuals, it is necessary to prepare correlation tables for the respective individuals for accurate measurement.
For example, Japanese Published Patent Application No. 2003-260041 (Patent Document 1) discloses a noninvasive type handy blood sugar level meter, wherein transmittance of light through ear lobule is measured as a biological feature amount, and blood that is collected at the same timing as the measurement timing is analyzed by an accurate analyzer to measure a blood sugar level, and then a correlation table showing correlation between the blood sugar level and the biological feature amount is prepared.
On the other hand, Japanese Published Patent Application No. 2000-37355 (FIG. 6) proposes a glucose concentration measurement method and apparatus adopting an eyeball as a measurement site, wherein a change of refractive index in aqueous humor is measured as a biological feature amount, and correlation between the refractive index and a glucose concentration in the aqueous humor is utilized. In such method and apparatus, when the correlation between the blood sugar level and the biological feature amount can be approximated by a primary straight line, a correlation table that is obtained by several points including low blood sugar levels and high blood sugar levels is prepared, whereby a blood sugar level can be estimated using the primary straight line if a biological feature amount that is not included in the correlation table is obtained.
FIG. 6 shows an example of a sugar tolerance test by sugar ingestion. In FIG. 6, five circles indicate measurement points that are obtained by performing invasive blood sugar measurement for every 30 minutes. The blood sugar level at time 0 minute is 100 mg/dl, and it increases to 140 mg/dl in 30 minutes, to 260 mg/dl in 60 minutes, to 300 mg/dl in 90 minutes, and then decreases to 280 mg/dl in 120 minutes. With reference to these five points of blood sugar levels, the time change curve of the blood sugar level can be approximated as a solid line shown in FIG. 6.
FIG. 7 shows a case where the relation ship between the blood sugar level and the biological feature amount can be approximated by a primary straight line. When the blood sugar level changes from 100 mg/dl to 300 mg/dl, the biological feature amount monotonically increases from 1010 to 1090. In the case where the blood sugar level and the biological feature amount can be approximated by a primary straight line as described above, if a correlation table between the five points of blood sugar levels and the corresponding biological feature amounts is prepared in advance, the blood sugar level can be estimated as 200 mg/dl and 225 mg/dl when the biological feature amount obtained by the noninvasive measurement is 1050 and 1060, respectively.
However, the biological feature amount that is obtained from a transmission amount, or an absorption amount, or a reflection amount of an electromagnetic wave, or an acoustic wave, or an optical wave which are applied to a biological body, or from spatial distribution of the transmission amount, or spatial distribution of the absorption amount, or spatial distribution of the reflection amount is adversely affected by multiple reflection/scattering, or interference. Therefore, usually, the biological feature amount does not change monotonically.
FIG. 8 shows the case where the relationship between the blood sugar level and the biological feature amount is not a primary straight line. In this case, since only a change in the biological feature amount as shown by a solid line in FIG. 8 can be estimated from the above-mentioned five points, the original change in the biological feature amount that is indicated by a broken line cannot be traced. For example, when the biological feature amount is 1050, the blood sugar level is estimated as 200 mg/dl although the correct blood sugar level is 180 mg/dl.
Of course, it is possible to accurately trace the change in the biological feature amount if plural blood sugar levels are previously obtained by performing a considerable number of invasive blood sugar level measurements. However, this method brings considerable pain on the subject.
In the conventional noninvasive blood sugar level measurement method, when the relationship between the blood sugar level and the biological feature amount is not the simple primary straight line, it is difficult to obtain an accurate correlation table. Further, performing many times of invasive blood sugar level measurements to obtain an accurate correlation table leads to an increase in the pain on the subject.